Time-delay circuit



May 30, 1950 s. H. DlKE TIME DELAY CIRCUIT Filed April 16, 1943 BY SHELD N .D/KE

Patented May 30, 1950 TET TIME -DELAY CIRCUIT Sheldon H. Dike, Vent-nor, N. J., assignor to the United States of America as represented by the Secretary of the Navy Application April 16, 1943, Serial No. 483,304

3 Claims.

This invention relates to thermionic control circuits and has particular reference to a novel time delay control circuit employing a thermionic tube and which is timed under control of a combination of resistance and capacitance of predetermined values. The new timing device is of simple construction and is reliable and accurate in operation.

The time delay circuit of the invention may be used for various purposes but because of its simplicity and accuracy it may be employed to particular advantage as a safety device for eX- plosive projectiles. Accordingly, fOr illustrative purposes, the invention will be described in the form of a time delay device for such projectiles, although it is to be understood that the invention is not limited to this use.

One object of the invention resides in the provision of a time delay device for use in explosive projectiles, which does not require the use of any moving part.

Another object of the invention is to provide a thermionic time delay circuit of novel construction, in which the time increment is controlled by the discharge of a condenser of predetermined capacitance through a predetermined resistance. In the preferred construction, the tube is of the hot-cathode gas-filled triode or tetrode type, in which conduction is normally prevented by the negative potential on the control grid of the tube. The negative potential is impressed upon the control grid by reason of the fact that the positive potential of a B battery is present at the cathode of the tube and the negative potential of the B battery is present at the control grid. The timing condenser is connected between the cathode of the tube and the control grid and is initially charged to full "B battery voltage. When a tetrode thyratron type of tube is used, the second grid is grounded. The device is operated by actuating a switching means which prevents further charging of the condenser and permits the condenser to discharge through the resistance. When the condenser has discharged suiiiciently to allow the potential on the grid to decline below a predetermined value, the tube becomes conducting so as to operate an electrical element, such as a detonating squib.

These and other objects of the invention may be understood by reference to the accompanying drawing, which illustrates one form of the new circuit for use as a safety device to detonate a squib in an explosive projectile at a predetermined time interval after the projectile is fired.

Referring to the drawing, the squib I, as shown,

is operable electrically by a radio apparatus comprising an amplifier 2 which may be connected to the output of an oscillator circuit of a proximity fuze (not shown), such as that disclosed in a ccpending application of M. A. Tuve et a1., Ser.

No. 471,388, filed January 6, 1943. The output of the amplifier is connected through a condenser 3 to the grid of a gas triode or thyratron 4. The grid is normally biased from the negative side of a 0 battery 5 through resistors 6 and 6a, the positive side of the battery being grounded, as shown at I. One side of the filament constituting the cathode of the tube 4 is connected through a terminal 8, wire 9, setback switch In, and ground connections II and I2 to the negative side of an .A battery I3. The positive side of the A battery is connected through a setback switch I4 and wire I5 to a terminal I6 which is connected to the other side of the filament. The anode of the tube 4 is connected by a conductor I! and a terminal I8 to one side of the squib I, the other side of which is connected through a setback switch I9 to the positive side of a B battery 20, usually of -100 volts, the negative side of the "13 battery being grounded at I2. The switches I0, I4 and I9 may be of any desired construction in which the contacts are normally open and are locked in the closed position by a force of setback incident to firing the projectile.

It will be understood that the grid biasing of the gas triode t is normally such as to prevent conduction in the tube and consequent operation of the squib I. When the projectile is fired from a gun, the resulting force of setback closes the switches I0 and It so as to connect the filament of tube 4 across A battery I3, and closes switch I9 so as to connect the anode through squib I to the positive side of the B battery. Accordingly, when a voltage pulse is delivered to the amplifier 2 from the oscillator (not shown) in response to proximity of a target, the pulse is amplified and acts to change the bias on the grid of tube 4 so as to cause the tube to become conducting, whereby the squib I is energized and explodes.

The time delay device of the present invention is illustrated in the form of a safety unit operable in conjunction with the radio controlled circuit previously described to cause detonation of the squib at a predetermined time interval after the projectile is fired, in the event that the radio controlled circuit fails to detonate the squib. The safety timing device comprises a second hotcathode thyratron 22 0f the triode or tetrode type, the filament of which is connected across terminals 8l6 in parallel with the filament of tube 4, the anode of tube 22 being connected by wire 23 to the terminal l8 and squib I. The

grid biasing circuit of tube 22 includes a con-,

tween the resistance 29 and the condenser, has

a negative charge of corresponding value. A condenser 3! is connected at one side between the resistors 6 and 6a and at the other side to wire 25.

When the setback switches l0, l4, and I9 close upon firing the "projectile, the filament of tube 22 'is connected across A battery 13, and the anode of tube 22 is connected through'squib l and switch IE! to the positive side of the *B battery. Due to the initial negative bias "on the grid of the tube 22, the tube is prevented from becoming conducting and energizing the squib. However, as soon as switch is closed, the charging circuit for the condenser 24 through resistor 29 ceases to act as such because wires 25 and 9 are now grounded, and the condenser commences to discharge through resistor 29 and the ground connection 28. When the condenser has discharged sufiicien'tly to cause the negative potential on the grid of tube 22 to decline below a critical value, the tube becomes conducting and causes a current flow through squib I from the anode of the "tube, with the resultthat the squib is exploded. The resistor 26 has a sufficiently high resistance that the load on BV battery 2!! is negligible when switch 10 is closed.

'It will 'be observed that the new device acts as a safety unit to insure explosion of the projectile in the event that the projectile does not pass sufficiently close to the target to cause detonation of the squib through amplifier 2. Thus, the device prevents damage which'might otherwise occur due to explosion of the projectile at the end of its flight on friendly territoryi The time interval provided by the control circuit is preferably so adjusted as to include the estimated time required for the projectile to'reach the target, in addition 'to a ffixed time interval, such as five :seconds, wherebyjthe timing circuit cannot explode the projectile until after the radio apparatus has hadan opportunity to "function through amplifier 2. The time interval may be adjusted byyarying the capacity of condenser 24 or the resistance of resistor 29. It has been found that by employing a resistor 29 of 'megohmsand a-condenser'24 of Armicrofarad, a time delay of approximately '27 "seconds is provided, although, of course, this specific delay is mentioned merely'as an'examplaand in no 'way as a limitation. It "will be "understood that the tubes 4 and 22 are made sufliciently 4 rugged to withstand the shocks incident to handling and firing a projectile.

What is claimed is:

1. In a detonator for an explosive projectile, a thyratron having a cathode, a control grid, and an anode, an electrically operable squib connected to the anode of the thyratron, a current source "in circuit with the squib and the anode, the grid being normally 'biased to secure the thyratron from becoming conducting, an amplifier connected to the grid and operable to change the bias thereof to cause the thyratron to become conducting and fire the squib, a second thyratron having a cathode, a control grid, and an anode, means connecting the anode of the second thyratron to the squib, a biasing circuit for said lastgridincludinga normally charged condenser for securing the second thyratron from becoming conducting, a resistor, and switching means responsive to firing the projectile for causing the condenser to leak through the resistor, whereby the second thyratron becomes conducting and fires the squib. V

'2. In an electrical self-destruction time delay devicegia thyratron having a cathode, a control grid and an anode, an electrically operable detonator including a squib and connected to said anode, a grounded current source connected in circuit with said cathode, said "anode, "and said detcnator, a condenser connected between the grid and the cathode, a resistor connected between the grid and the ground, means for charging the condenser irom said source throughs'aid resistor, and means for rendering said "charging means inoperative and causing the condenser to leak through the "resistor; whereby the thyratron becomes conducting and fires the squib.

3, In an electrical self-destruction time delay device for an explosive projectile, a thermionic gas-containingtube having a cathode, a control grid, and an anode, an electrically operable detonator, including a squib and connected to said anode, a current source in circuit with fthe'deto'na'tor and the anode and cathode; a biasing circuit connected to "the grid and cathode and including a normally charged condenser for securingth'e tube a ainst flashing, a resistor, said resistor having one terminal permanently con The following references are of recordin the file of this patent:

' UNITED STATES PATENTS Number *Name Date 1,552,321 Lea "Sept. 1, 1925 1355;023 'Ruhlem'ann Apr. 15, 1930 2,341,351 "Barkley Feb. 3;1944 

